1. Field of the Invention
The invention relates to an AC to DC power converter, and more particularly to an AC to Dc power converter having hold-up time function.
2. Description of Related Art
With reference to FIG. 4, a conventional power converter for converting a high AC voltage to a low DC voltage is disclosed in U.S. Pat. No. 6,504,497. The power converter comprises a full wave rectifier (50), a power factor correcting (PFC) circuit (51) a switch type DC to DC converter (52), a capacitor (53) and a hold-up time extension circuit (54).
The full wave rectifier (50) receives an AC power with the peak voltage ±220×1.141≅±310V and converters the AC power to a sine DC power with the peak voltage +310V.
The PFC circuit (51) is connected to the full wave rectifier (50) to receive the sine DC power and regulates a voltage phase and a current phase of the sine DC power to be consistent with each other. The regulated DC power is further input to a voltage boost circuit for producing a high DC voltage (+380V).
The switch type DC to DC converter (52) is connected to the PFC circuit (51) to receive the high DC voltage. The switch type DC to DC converter (52) converts the high DC voltage to a required DC voltage with a relative low level such as +12V or +5V.
The PFC circuit (51) is constructed by a voltage boost circuit and generates a voltage with a higher level than the sine DC power output from the full wave rectifier (50). Since the power converter has to supply a low DC voltage, the switch type DC to DC converter (52) accordingly bucks the high DC voltage. For example, when the switch type DC to DC converter (52) is operated in the continuous conduction mode (CCM) to convert the high DC voltage of 380V to a relative low DC voltage of +12V or +5 V, an active switch of the switch type DC to DC converter (52) has the duty ratio D=Vo/Vi. According to the equation, the duty ratio of the active switch is small and the power conversion efficiency is low.
Furthermore, to ensure that the load can obtain an emergency operating voltage for safely shutting down once the AC power is suddenly interrupted, the power converter further includes the hold-up time extension circuit (54) and the capacitor (53). The capacitor (53) is coupled to an output capacitor (511) through the hold-up time extension circuit (54). When the voltage of the output capacitor (511) is lower than a threshold value, the capacitor (53) provides power for the hold-up time extension circuit (54). The hold-up time extension circuit (54) converts the power from the capacitor (53) to a DC voltage and further supplies the DC voltage to the switch type DC to DC converter (52) and sustains the voltage level on the output capacitor (511) to extend the hold-up time.
As described above, the conventional power converter has to add a hold-up time extension circuit having the boost circuit configuration to extend the hold-up time. Therefore, the power converter accordingly includes two voltage boosting circuits that respectively need a separate PWM controller (IC1)(IC2). As a result, the power conversion efficiency is low and the circuit design is complex and expensive.
To overcome the shortcomings, the present invention uses an AC to DC power converter with hold-up time function to mitigate or obviate the aforementioned problems.